Partial refactor of the Snapshot tool

[paulthatjazz]

It can be a bit slow at times and needs some polish.

• Addition of caching so data isn't recalculated every time the layer changes.
• Multi threading, possibly, to improve performance
• General tweaks to calculations to ensure efficiency

Some other, less significant issues:

• Histogram when min = max does not look great.
• Zero values may need further consideration.